One method used to fabricate superconducting wires with multi- and mono-filament composite conductors is the Wind-And-React (WAR) process. In this process, the eventual conducting material is typically considered to be a “precursor” until after a final heat-treating and oxidation step. The WAR method as applied to high temperature superconductors requires that the precursor be insulated before coil formation, and entails winding the coil immediately prior to a final heat-treating and oxidation step in the fabrication process. The WAR process as applied to high temperature superconductors requires that the precursor be wound in coil-form for high-field magnet application prior to the final heat-treatment. This final step results in the repair of micro-cracks incurred during winding, and is used to optimize the superconducting properties of the conductor. However, these results are significantly more difficult to achieve for a coil geometry than for the individual wires which are heat-treated and oxidized in a React-And-Wind (RAW) method. The RAW method, involves heat-treating the wire prior to coiling.
The RAW method involves the formation of a superconductor precursor which is then wound into a coil. In this method, a precursor to a composite conductor is fabricated and placed in a linear geometry, or wrapped loosely around a spool, and placed in a furnace for processing. The precursor can therefore be surrounded by a vacuum or inert gas environment during processing, which is necessary for conversion to the desired superconducting state. In the RAW processing method, insulation can be applied after the composite conductor is processed. In some cases, the RAW process can be advantageous in that it does not suffer from issues such as the oxygen permeability and thermal decomposition of the insulating layer.
Both WAR and RAW processes have their own merits. The WAR process is advantageous when the material in its superconducting state is brittle because it limits handling. However, the WAR process is expensive for large coils, partly, due to a necessary epoxy vacuum impregnation process for interstrand electrical insulation as well as associated furnace costs. The RAW process can be advantageous in terms of cost because insulation can be applied before coiling and smaller furnaces can be used when producing large coils. Another advantage of the RAW process is that mechanical damage in the strand can be inspected for and repaired during the coiling process. Due to the mechanical properties of the conducting material, superconducting magnetic coils fabricated using the WAR method with mono-strand composite conductors have limitations related to winding density and current-carrying ability. Although the final step of the WAR method may repair strain-induced damage to the superconducting material incurred during winding, the coils produced are not mechanically robust, and thermal strain resulting from cool down cycles can degrade the coil performance over time.
An important issue for the fabrication of superconducting coils is the strain state of the conductor. Reaction heat-treatment of a superconducting precursor to form the superconducting phase causes the superconductor to undergo dimensional changes and introduces strain to the superconductor. For example, in Nb3Sn conductors, dimensional changes in an unconstrained superconductor under a heat-treat cycle can include: (1) changes during annealing due to stress relief; and (2) changes due to the formation of Cu—Sn intermetallics and finally formation of Nb3Sn phase. For strand conductors, the residual stresses after the drawing process are proportioned such that the niobium filaments are under tension while the copper matrix is under compression. At 200° C. the copper matrix begins to stress relieve and soften allowing the niobium filaments to relax via contraction resulting in permanent contraction of the strand. Two mechanisms can mitigate the contraction of the strand. Firstly, for wire with physically constrained ends, the niobium will maintain a tension stress state until it undergoes stress relief at 650° C. resulting in permanent deformation of the niobium component and an elongation contribution to the strand when cooled back down to room temperature. Secondly, since 1 mole of Nb3Sn has a larger volume than 3 mole niobium and one mole of tin some expansion does take place partially negating the contraction. The elongation contribution creates a problem because in order to avoid strand breakage during heat-treatment due to the stress relief contraction the strand must be spooled with minimal tension applied, but due to the expansion contribution, sufficient pay-off tensions cannot be realized. In some cases, an interlayer gap can form resulting in strand damage during pay-off due to strand bending. The contraction of the strand can be avoided by pre-heat-treat stress relief, but expansion can still occur due to the formation of the Nb3Sn phase.
Wire-in-channel (or cable-in-channel) superconductors are often passed through a die to press a superconductive material into a groove of a channel. The die may also serve to draw the conductor to a final cross-section, and in doing so may deform the channel to more securely retain the superconductive material in the channel. However, due to the brittle nature of the superconductive material, deforming the channel can apply stresses to the superconductive material.
Exceeding the critical strain value of the superconductor wire results in severe degradation of the electrical properties of the superconductor. Therefore, handling a superconductor precursor after reaction in the RAW process can be difficult.